feat(bff): POST /self-service/registrations/{id}/documents (S-10a) (refs #102)

Authenticated self-service endpoint that takes the bsn from the DigiD token,
forwards "documenten aanleveren" to the domain, and relays 404 for an unknown or
not-owned registration. Regenerates the committed openapi.json.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/bff/openapi.json

@@ -61,6 +61,37 @@
         }
       }
     },
+    "/self-service/registrations/{id}/documents": {
+      "post": {
+        "tags": [
+          "Bff.Api"
+        ],
+        "parameters": [
+          {
+            "name": "id",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "204": {
+            "description": "No Content"
+          },
+          "400": {
+            "description": "Bad Request"
+          },
+          "401": {
+            "description": "Unauthorized"
+          },
+          "404": {
+            "description": "Not Found"
+          }
+        }
+      }
+    },
     "/openbaar/register": {
       "get": {
         "tags": [